PHYSOR 2004 -The Physics of Fuel Cycles and Advanced Nuclear Systems: Global Developments Chicago, Illinois, April 25-29, 2004, on CD-ROM, American Nuclear Society, Lagrange Park, IL. (2004)
The TRADE Experiment: Status of the Project and Physics of the Spallation Target
C. Rubbia1, P. Agostini1, M. Carta1, S. Monti1, M. Palomba1, F. Pisacane1, C. Krakowiak2, M. Salvatores2, Y. Kadi*3, A. Herrera-Martinez3, L. Maciocco4 1ENEA, Lungotevere Thaon di Revel, 00196 Rome, ITALY 2CEA, CEN-Cadarache, 13108 Saint-Paul-Lez-Durance, FRANCE 3CERN, 1211 Geneva 23, SWITZERLAND 4AAA, 01630 Saint-Genis-Pouilly, FRANCE
The neutronic characteristics of the target-core system of the TRADE facility have been established and optimized for a reference proton energy of 140 MeV. Similar simulations have been repeated for two successive upgrades of the proton energy, 200 and 300 MeV, corresponding to different performances and design requirements and different characteristics of the proposed cyclotron and, as a consequence, of the proton beam. An extensive comparison of the main physical parameters has been also carried out, in order to evaluate advantages and disadvantages of different proton beam energies in the design of the spallation target and to allow the optimal engineering design of the whole TRADE facility.
KEYWORDS: TRADE facility, ADS, Spallation target physics, Experimentation, External neutron sources, intermediate proton energy.
1. Introduction
The TRADE “TRiga Accelerator Driven Experiment”, to be performed in the existing TRIGA reactor of the ENEA Casaccia Centre, has been proposed as a major project in the way for validating the ADS concept. Actually, TRADE will be the first experiment in which the three main components of an ADS – the accelerator, the spallation target and the sub- critical blanket – will be coupled at a power level sufficient to appreciate feedback reactivity effects. As such, the TRADE experiment represents the necessary intermediate step in the development of hybrid transmutation systems, its expected outcomes being considered crucial – in terms of proof of stable operability, dynamic behavior and licensing issues – for the subsequent realization of an ADS Transmutation Demonstrator. As already shown in previous papers [1,2], the experiments of relevance that can be performed in TRADE concern: ∑ the dynamic system behavior of an ADS vs. the neutron importance of the external source at different sub-criticality levels, thus providing important information on the optimal sub-criticality level; ∑ Sub-criticality measurements at significant power; ∑ Correlation between reactor power and proton current; ∑ Reactivity control (neutron source importance method); ∑ Compensation of power effects of reactivity swing with control rods movements or with proton current variation; ∑ Start-up and shutdown procedures, including suitable techniques and instrumentation.
* Corresponding author, Tel. +41-22-767-9569, FAX +41-22-767-7555, E-mail: [email protected] In this paper, we will present the status of the project, as well as results of recent simulations and analyses concerning the physics of the spallation target for different values of the proton energy, from 140 MeV up to 2-300 MeV.
2. Status of the Project
The TRADE experiment - aimed at a global demonstration of the ADS concept - is an original idea of Carlo Rubbia developed through a feasibility study carried out by an ENEA and CEA Working Group over 2001-beginning of 2002. These feasibility studies have been followed by further conceptual design activities performed in 2002-2003 by an International TRADE collaboration set up by ENEA (Italy), CEA (France), FZK (Germany), DOE (USA), CIEMAT (Spain), CNRS (France), AAA (France), and ANSALDO (Italy). The overall layout of the facility - selected after a quite comprehensive comparison among others - is shown in Figure 1 [3]. It foresees the erection of a new bunker, close to the existing TRIGA building, to house the accelerator and the test station for proton beam test and adjustment. The proton beam is transferred from one building to the other via a section of the transfer line that is particularly simple, since the cyclotron is at the same level of the top of the reactor. The beam transport line is protected by a massive shielded tunnel which extends into the TRIGA building up to the reactor top. Through the straight section of the transport line, the beam is transferred to the final bending section composed of two magnetic dipoles and three magnetic quadrupoles, which have the duty of directing the beam, with the correct size, to the spallation target placed in the central thimble of the reactor.
Fig.1 Reference layout of the TRADE facility (vertical section)
The studies performed so far by the TRADE International Collaboration have concerned: ∑ The neutronics of selected possible configurations, along with a neutronic benchmark to define codes and tools to be used for the neutronic design and the interpretation of the experimental measurements; ∑ The core and target thermal hydraulics, using both natural or forced convection, including the target coupling to the reactor at power; ∑ The target performances and characteristics as well as the conceptual design of the target, its cooling system and the definition of the tests needed for its qualification; ∑ A conceptual design of the Beam Transport Line; ∑ The shielding and activation aspects in order to gain insight on the dose issues. ∑ The safety and licensing aspects related to the plant modifications induced by the TRADE experiment, including considerations on general safety criteria, possible accident initiators and a preliminary hazards analysis; ∑ The overall experimental program to be performed in TRADE; ∑ The representativity of the foreseen experiments in terms of dynamic behaviour, neutron spectrum, reactivity control, proton current/power relation, operation at start- up and shut-down, external source importance measurements, etc.. ∑ A preliminary cost and time schedule evaluation. Furthermore, a preliminary experimentation in the TRIGA RC1 reactor was carried out in fall 2002, to characterize the TRADE reference core; a new experimental phase - which will include measurements in a mock-up of the TRADE core with Californium, DD and DT external sources – is being performed over 2003-2004 (reported in another paper at this conference). As for the accelerator, several configurations were studied for a suitable proton energy beam of 140, 200 and 300 MeV and with a beam current in the range 100-300 µA. Taking into account the constraints related to the cost and the relatively short time scale for the implementation of the TRADE experiment, a room temperature H- cyclotron is considered the most affordable solution. Actually, even if the requested performances for TRADE (140 MeV, 2-300 µA) are rather challenging with respect to the ones of the existing machines, the TRADE cyclotron can be regarded as an evolution of a typical H- machine for radioisotope production (Ep around 30 MeV) or for hadron-therapy (Ep around 60 MeV). As for international agreements to implement the TRADE experiment, a Memorandum of Understanding (MOU) among “funding” partners - ENEA, CEA, DOE and FZK – was signed in 2003 by ENEA, CEA and FZK. TRADE-PLUS – the part of the TRADE project devoted to the design of the facility as well as to the experiments and their interpretation and transposition to the future ADS Demonstrator - is also one of the major subprojects of the Integrated Project EUROTRANS, which is being presented to the European Commission by several European associations within the EURATOM 6th Framework Programme in the Thematic Priority Area “Management of Radioactive Waste: Transmutation”.
3. Physics of the Spallation Target
The spallation target is the key component of any ADS concept. Even in the TRADE facility, despite the relative small power of the proton beam (<40 kW), the development and design of the target implies a detailed assessment of different aspects mutually interacting, from the physics of spallation reaction - including neutron generation and distribution, spallation products yields and damage rates – to technological issues, such as choice of the most suitable material, power density distribution, heat removal, thermo-mechanics, fabricability, etc. In particular, accurate and rigorous assessment of nuclear parameters under different physical conditions is the prerequisite for an optimal design of the target and its interaction with the (subcritical) TRIGA core. This work aims at evaluating, by probabilistic transport codes (FLUKA and EA-MC), the main neutronic and physical parameters such as yield and energy and angular distribution of the spallation neutrons, proton and neutron flux around the target, energy deposition, radiation damage, spallation product yields and radioactivity. The calculations have been performed for different intermediate energies (140, 200 and 300 MeV) of the proton beam impinging on a solid Tantalum target, allowing the evaluation of pros and cons of different solutions as well as the most suitable cyclotron for the TRADE facility and its successive utilizations. The performances and the impact on the target design of different shapes of the proton beam and geometrical configurations of the spallation target have been also assessed. The guidelines which have been followed for the target development together with the main constraints and interfaces have been extensively discussed in ref. [4]. The geometry is described below to provide justification of the choices in relation with the aforementioned design elements.
3.1 Geometrical description of the target The draft prototype described here corresponds to configuration #10 (Figure 2).
Fig.2 Target configuration #10
3.1.1 Inner geometry The inner geometry (Figure 2) is characterized by three conical cavities having different angles and total length equal to the active height of the TRIGA core. The cone tip (lowest cone) is exposed to the highest power density for two reasons: ∑ the relevant proton current at the centre of the Gaussian distribution, ∑ the forward scattering of protons as a consequence of the conical angle steepness. Moreover, the deposited power at the tip of the cavity is very sensitive to the reduction of the sigma value of the gaussian beam, as shown in Figure 3 for 140 MeV proton energy. To better distribute the power, it is necessary to work out a small diameter at the extremity of the cone. The former shape of the cone tip presented a series of drilled cylindrical bores which are now substituted by a smooth conical surface. This improvement was demonstrated to be feasible by the spark erosion technique up to a diameter of 1.5 mm as shown in Figure 4. ) A m * 3 m c ( / w
y t Power density at cone tip vs. sigma
i 40 s n e 35 d 30 r e w 25 o p 20 7 7.5 8 8.5 9 Sigma of Gaussian (mm)
Fig.3 Power density at cone tip Fig.4 Spark erosion bores in Ta
The conical wall angle of the lower cone (“Dett.C” in Figure 2), having predetermined the larger diameter as 6 mm, is identified by its length: the larger the length, the smaller is the linear power; nevertheless a maximum limit of 80 mm can be identified for fabrication reasons. The smooth conical surface strongly improves the power distribution of the lower cone which is more uniformly heated up. It is evident that, by increasing the cone length, one can decrease the power distribution at the tip, but this procedure implies an increase of linear power at the intermediate cone which should be balanced to better distribute the power in the whole spallation volume. Elastic thermo-mechanical calculations show that a best compromise can be reached with a lower cone length of 60 mm [5]. The intermediate truncated cone (“Dett.B” in Figure 2) has an angle of 7.35° and receives the largest amount of power (77% of the total). In this region the forward scattering of protons mainly occurs, therefore, in order to widen its angle, a conical region having steeper angles was located below its position. The upper truncated cone (“Dett.A” in Figure 2) is very short (21 mm) and has the function to directly connect the target to the beam transport line; the tails of the Gaussian profile are here truncated. This cone has the largest angle to keep constant the axial position of the beginning of the spallation process, even in presence of small radial errors. The impinging power is relatively low as shown by the calculated axial distribution of power (section 3.4.5).
3.1.2 Outer geometry The typical range of 140 MeV protons, being stopped in Tantalum by electronic interactions, can be approximately expressed as:
1.75 ER(E) = E 5811 where ER(E) is the electronic range (cm) and E is the proton energy (MeV). The approximated electronic range was 9.8 mm and a thickness of 15 mm was assumed in the design as the best compromise to assure the radial heat removal without intolerable thermal stresses. At the lowest cone the 15 mm thickness produces excessive thermal stress and has to be reduced. This is due to the shape factor “1 ln(re ri)” which affects the integration of Fourier Law in axial-symmetric geometries. In Figure 5 the comparison between the shape factors, is reported for the lowest cone in case of constant thickness and reduced thickness. ) l a n o i s n e m i d a (
r Lowest cone shape factor o 0.6 t c a 0.53 f
e 0.45 p a 0.38 h s 0.3 0 15 30 45 60 lowest cone length (mm) constant thickness reduced thickness
Fig.5 Variation of the shape factor versus lowest con length
The upper part of the cavity presents a thickness smaller than 15 mm because of the outer geometrical constraint: 63 mm maximum diameter.
3.2 Thermal performances of the target In presence of the design mass flow-rate of water (2.24 kg/s), the maximum thermal flux at the outer wall of the target is 135 W/cm2 (Figure 6) thus assuring a margin large enough to prevent the occurrence of Critical Heat Flux. Moreover the maximum temperature is 80°C (Figure 7), which is significantly lower than the TRIGA saturation temperature.
Fig.6 Wall thermal flux Fig.7 Outer wall temperature The thermal performances of the target configuration #10 under the reference operating conditions are reported in Table 1.
Table 1 Operating conditions
The velocity and temperature fields are reported in Figure 8.
Fig.8 Velocity and Temperature fields for the target configuration #10
This target configuration presents a better temperature distribution which comes out of a more balanced power density in the target material, as evidenced by the lower temperature peak.
3.3 Mechanical resistance of the target The elastic stress calculations reported in [5] show a relevant stress value, which is higher than the yield limit of the material (100 Mpa at 300ºC). Since the thermal loads are secondary loads, they progressively disappear as soon as plastic deformations take place; this working condition is defined as elastic-plastic. In nuclear applications the elastic-plastic condition of structural materials is generally accepted provided that some design rules are respected. The acceptability rules are mainly connected with the alternate nature of the loads; in particular two phenomena have to be studied in our case: progressive plastic deformation (or ratcheting) and fatigue. Due to the operating temperature that is lower than 815°C, the creep effects can be neglected for Tantalum. Calculations of plastic strain on the target configuration #10 when exposed to 40 kW beam power, by 140 MeV protons, are reported in Figures 9a (upper target) and 9b (lower target).
Fig.9a Plastic strain of the upper target Fig.9b Plastic strain of the lower target
The corresponding total strains are reported in Figures 10a and 10b.
Fig.10a Total strain of the upper target Fig.10b Total strain of the lower target
The maximum strain (1.45 %) is located in the stepped region while a lower local maximum (1.12 %) corresponds with the region of maximum temperature. The actually preferred solution relies on a smooth lower surface (configuration #10), which will reduce the highly stressed value of the stepped region [5]; nevertheless some rough fatigue considerations can be drawn. In Figure 11 a series of calculated plots of the behaviour of Tantalum and Tungsten at room temperature under strain controlled fatigue is reported [5]; the plots are obtained after application of the Manson-Coffin numerical procedure to literature results obtained by load controlled tests. It seems that Tantalum can survive a total strain range of 1.4 % after 1000 cycles. The results have to be confirmed by more direct experiments at operational temperature and are presently considered only a reference indication. In order to enhance the target duration a lower level of proton beam power must be envisaged. Fatigue Strain of W and Ta ) 10 % (
n i a r t s
l a t 1 o t
0.1 3 4 5 6 1 .10 1 .10 1 .10 1 .10 cycles to rupture Tungsten annealed at 1480°C, tested at room temperature Tantalum in plate annealed at 1400°C,tested at room temperature Tantalum in wire annealed at 1400°C, tested at room temperature
Fig.11 Calculated fatigue strain of Ta and W at room temperature
3.4 Nuclear performance of the target The main neutron physic parameters of the target such as neutron yield and energy spectra, power deposition, material damages and spallation product distributions are evaluated by probabilistic transport codes (FLUKA/EA-MC and MCNPX). The impact of different target parameters (material choice, geometry of the proton beam, energy of the protons) has been studied extensively [6,10]. While the neutron yield and spectra are mainly related to the nuclear behavior of the system, energy deposition is directly related to the thermo-mechanics of the target and its cooling capabilities, which determine its lifetime in the core.
3.4.1 Neutron production The main goal of the spallation target is the neutron production. Calculations performed for 140 MeV protons distributed according to a Gaussian profile (as discussed in section 3.1.1) on thick (configuration #1 and #10) and thin (configuration #4) geometries of the Tantalum target show that the neutron yield (Table 2) is not affected significantly by the target geometry since the protons are almost at the end of their range when they leave the target. Even in the “thin” geometry, the material thickness (3 mm in the radial direction) is sufficient to allow the whole spallation reactions to take place inside the target’s material.
Table 2 Neutron production process Target type Neutron yield Thick target (config. #1) 0.80 n/p Thin target 3 mm thick (config. #4) 0.79 n/p Thick target (config. #10) 0.75 n/p
The neutron flux distribution for configuration #10 is reported in Figure 12. Since the region of maximum production lies below the median plane of the core, it is necessary, in order to achieve a better efficiency in the use of the source neutrons, to axially shift, by about 4 cm, the target body in the upward direction. Fig.12 Neutron flux distribution in (n/cm2/s) per kW of beam
The neutron spectra shown in Figure 13 are the result of a complete simulation performed with the target described above, inserted in the central channel of the TRIGA core where the fuel pins have been removed (no contribution by the core thermal neutrons). In reality the contribution by the core thermal neutrons is not negligible and will result in additional activation of the spallation target. For high energies, it is possible to distinguish the well- known peak at 1 MeV related to evaporation phase of the spallation process. A second broader peak is clearly distinguishable at higher energies (above a few tens of MeV), which is related to the INC phase (Intra-Nuclear Cascade) of the spallation process.
Fig.13 Neutron flux spectra at several locations inside the spallation target vessel in (n/cm2/s) per kW of beam. The average energy of the high-energy neutrons (>20MeV) is also reported
Overall, the integrated flux of neutrons escaping the target (i.e. entering the core) is reduced by a factor 10 as a result of the successive attenuation in the target body but also in the thick shroud surrounding it. For neutrons above a few 100 keV’s, the attenuation reaches almost a factor 100. Nevertheless, the high-energy tail of the neutron distribution when entering the core inner structures is still present and the high-energy neutrons (> 10 MeV), only slightly moderated, still represent 4.5% of the spallation neutron population with an average energy of about 45 MeV. This fraction was of the order of 7.5% inside the spallation target with an average energy of 51 MeV, as shown in Table 3. Table 3 Neutron flux spectra Neutrons escaping Neutrons escaping Energy groups from vessel from target (% of flux) (% of flux) 0. – 1. eV 34.0 0.6 1. eV – 1. keV 10.5 0.8 1. keV – 1. MeV 31.5 51.9 1. MeV – 10. MeV 19.5 39.7 > 10. MeV 4.5 7.0
These high-energy neutrons are very hard to shield, and contribute to a certain extent to the radiation damage of the inner structure of the core but also to the ambient dose in the TRIGA reactor building should they leak out. Moreover, they will most probably react with the core coolant and produce N16 through (n,p) reactions on O16 which has a threshold at ~ 10 MeV and which cross section peaks at 0.15 barn compared to the 0.2 barn total cross section of H. This is further illustrated in Figure 14, which reports the spatial distribution of the high- energy component (> 20 MeV) of the neutron flux in the TRADE core.
Fig.14 Neutron flux distribution of the > 20 MeV in (n/cm2/s) per kW of beam
We estimate the flux of neutrons > 20 MeV reaching the inner most fuel elements to be of the order of a few 109 n/cm2/s per kW of beam and up to 107 in the bottom part of the reactor vessel. These neutrons tend to be forward peaked (> 130º), contrary to the low-energy neutrons which are more or less centred along the core mid-plane (Figure 12). On the other hand, any beam offset will produce an axial shift of the high-energy neutron distribution that can be easily monitored by placing at several locations along the height of the target a series of detectors sensitive to the recoils generated by the high-energy component of the neutron flux (> 1 MeV). These high-energy neutrons penetrate deep inside the reactor core and surrounding biological shield (Figure 15) and very few leaks out into the reactor building, without fortunately contributing to the ambient dose therein, as shown in Figure 16. Fig.15 Radial distribution of the high-energy Fig.16 Ambient dose due to neutrons in component (> 20 MeV) of the neutron flux in (mSv/h) per mA of beam (n/cm2/s) per kW of beam
3.4.2 High-energy proton leakage The release of protons out of the spallation target causes the production of unwanted radionuclides in the cooling water, therefore it has to be taken into account to identify the target acceptability and if possible minimized. The primary proton flux distribution for the target configuration #10 is reported in Figure 17. The fraction of protons escaping the target vessel is almost negligible apart from the upper part of the target (direct connection to the beam transport line) where the tails of the Gaussian profile are truncated. In any case the majority of the protons escaping the spallation target are either stopped in the cooling channel (riser) surrounding the target or inside the thick flow guide, none reach the core internal structures and only a few are backscattered into the vacuum beam pipe.
Fig.17 Primary proton flux distribution in (p/cm2/s) per kW of beam
Figure 18 reports the spectra of protons (both primary and secondary) escaping from the target and from the target vessel. The proton flux above 10 MeV escaping from the target vessel, has been reduced by almost three orders of magnitude. The high-energy tail, which is still present, but at a much reduced scale, corresponds to those protons cut out from the tail of the Gaussian profile at inlet. The low-energy proton flux results mostly from high-energy neutron (n,Xp) reactions in oxygen and in the structural material, but also from low-energy neutron elastic collisions on hydrogen, as shown in Figure 19. This is clearly illustrated by the fact that the spatial distribution of the secondary protons emerging from the target coolant is very similar to that of the high-energy neutrons, plotted in Figure 14.
Fig.18 Proton flux spectra at several locations Fig.19 Primary and secondary proton flux inside the spallation target vessel in (p/cm2/s) distribution in (p/cm2/s) per kW of beam per kW of beam
3.4.3 Residual products and activation In the target cooling channels (down-comer and riser) as well as in the inner most ring of the reactor (ring C), spallation and activation products (no contribution from the core neutrons) are released in the water; their amount was calculated and reported in Figures 20a and 20b.
Fig.20a Yield of residual products in water Fig.20b Yield of residual products in water vs. A in (nuclei/cm3/s) per kW of beam vs. Z in (nuclei/cm3/s) per kW of beam
The volume of water contained in the target cooling channel (riser part) is approximately 338 cm3, therefore, according to the calculations, the maximum production of the most problematic isotopes after one year of operation (2000 hours) is given in Table 4. Table 4 Residual yield of the most troublesome isotopes in the target cooling channel in (g/year) per kW of beam Isotopes Riser Down-comer H-3 8.70 10-9 2.43 10-8 Be-7 9.40 10-9 2.25 10-8 C-14 5.40 10-8 1.72 10-7 N-16 1.33 10-7 6.05 10-7
Burn-up calculations have been carried out to estimate the activation of the Tantalum spallation target. As shown in Figure 21, it is worth noting that the activity of the spallation target (expressed in Ci per year of irradiation, i.e. 2000 hours, per kW of beam) is dominated by activation resulting from successive thermal neutron captures during the first year. After one year the activity is dominated by the decay of the spallation products, mostly Hf, Lu and Yb isotopes. At longer times (> 40 years) tritium is the only isotope of importance. Note that tritium is the only troublesome volatile produced in the spallation target.
Fig.21 Evolution of the radioactivity of the Tantalum spallation target as a function of time in (Ci/yr) per kW of beam
3.4.4 Radiation damage The particle flux spectra generated by FLUKA [11] can be used to estimate the heating and damage to structural materials by protons and neutrons with energy above and below 20 MeV. Indeed, in TRIGA one can consider separately the high-energy portion of the spectrum, due to the primary proton shower, with its intensity proportional to the beam current, and a lower energy region associated with the fission-multiplying medium, proportional to the reactor power. In practice, we have evaluated: (dpa) E d ·s a Ò 10-21 [dpa/s] Eq.(1) = 2E ◊f ◊ dt d h E E where ·s a Ò is the damage energy production cross section (barn-keV), d is the energy required to displace an atom from its lattice position (eV), h = 0.8 is the collision efficiency factor and f is the particle flux (cm-2.s-1). As regards the damage induced by high-energy particles (> 20 MeV), we have used data that provides values for proton- and neutron-induced displacement cross sections as calculated using the default physics models in MCNPX [12] and illustrated in Figure 22.
Fig.22 Neutron and proton-induced damage-energy cross sections for Tantalum in (barn-keV)
Table 5 indicates the values of the integrated flux in the spallation target unit, due to high- energy particles (HE).
Table 5 Integrated flux per kW of beam power for different target configurations
HE proton Flux HE neutron Flux Ta Target (part/cm2.s) (part/cm2.s) Max Ave Max Ave 110 MeV SOL-1 4.0x1013 7.7x1012 8.4x1010 1.3x1010 140 MeV SOL-7 4.7x1013 6.7x1012 2.1x1011 1.7x1010 140 MeV SOL-10 C4 2.9x1013 3.6x1012 1.6x1011 3.1x1010 140 MeV SOL-10 C200 1.8x1013 2.7x1012 1.1x1011 2.5x1010
The gas production and the displacement rates (dpa/yr) obtained using Eq. (1) together with the particle fluxes listed in Table 5 are reported in Table 6 for the same operating conditions as those listed above, that is per kW of beam and for an expected duty factor during the TRADE experiments of about 22% (2000 hours).
Table 6 Gas production and the displacement rates per kW of beam Average Average H He Target 3 HE proton HE neutron Prot. Ener Neut. Ener Production Production (Ta) (dpa/yr) (dpa/yr) (MeV) (MeV) (appm/dpa) (appm/dpa) Max Ave Max Ave SOL-1(110MeV) 60 46 1.45 58.1 1.1 0.14 0.002 0.000 SOL-7 90 50 0.40 25.0 1.5 0.16 0.004 0.000 SOL-10 C4 90 51 2.01 165.8 0.9 0.09 0.003 0.001 SOL-10 C200 90 51 0.99 54.8 0.6 0.07 0.002 0.001 The particle damage are entirely dominated by the high-energy protons and localized on the inner skin of the spallation target with a maximum situated at the tip of the inner cone. It is further reduced (x 2) when the proton energy increases to 140 MeV.
3.4.5 Energy deposition While the previous parameters were mainly related to the nuclear behaviour of the system, energy deposition is directly related to both the thermo-mechanics of the target and its cooling capabilities, which determine its lifetime in the core. The power density distribution, or the distribution of the heat deposited, is a central factor in the thermal design of spallation targets. The largest value of the power density, which is equal to 175 W/cm3 per kW of beam of 140 MeV protons, is found at the tip of the conical cavity at the bottom of the target as shown in Figures 23a and 23b.
Fig.23a Power deposition inside the target Fig.23b Power deposition in the tip region in (W/cm3) per kW of beam at 140 MeV in (W/cm3) per kW of beam at 140 MeV
The distribution inside the target reveals a rather homogenous behaviour with an average power density of ~ 4 W/cm3 per kW of beam. A discontinuity is observed in the aluminium beam pipe (where the tail of the Gaussian profile is cut out) and in the upper truncated cone region of the target where the cone has the largest angle to keep constant the axial position of the spallation neutron source distribution, even in presence of small beam offsets. However, the impinging power there is relatively low as shown by the calculated axial power distribution reported in Figure 24.
Fig.24 Linear power distribution in (W/cm) per kW of beam for different target configurations and beam profiles 3.5 Application of higher proton energies to the reference target configuration The neutronic characteristics of the target-core system of the TRADE facility have been optimized for a reference proton energy of 140 MeV. Similar simulations have been repeated for two successive upgrades of the proton energy, 200 and 300 MeV, corresponding to different performances and design requirements and different characteristics of the proposed cyclotron and, as a consequence, of the proton beam. In this section, an extensive comparison of the main physical parameters is carried out, in order to evaluate pros and cons of different proton beam energies in the design of the spallation target and to allow the optimal engineering design of the whole TRADE facility.
3.5.1 High-energy proton leakage The fraction of protons escaping the target vessel increases with the proton beam energy. On average between 109 and 1011 particles per kW of beam manage to reach the first fuelled region. Their average energy is of the order of 30 MeV and reaches up to 65 MeV when the beam proton energy is increased to 200 and 300 MeV respectively. They are in any case all stopped in the core. In Figures 25 and 26, the proton leakage from the spallation target unit is represented for initial proton energies of 200 and 300 MeV respectively.
Fig.25 Primary proton leakage from the target unit in (p/cm2/s) per kW of beam at 200 MeV
Fig.26 Primary proton leakage from the target unit in (p/cm2/s) per kW of beam at 300 MeV
The induced radioactivity of the target cooling circuit increases when the proton energy is increased since protons are escaping more readily out of the spallation target and are undergoing more nuclear interactions in the water surrounding the target. Figure 27 reports the production of the light isotopes in target cooling water. Figures 28a and 28b report the activation of the target and its cooling circuit by the generated spallation products. Assuming the water volume in the riser is approximately 338 cm3, one can calculate the water activation by the protons. The latter reasoning is only applicable to protons up to 200 MeV, because at higher energies the complete stopping of protons in the flow guide seems unfeasible by the material thickness of the present target configuration. Fig.27 Yield of residual products in the target cooling circuit in (nuclei/cm3/s) per kW of beam at different incident proton energy
Fig.28a Activation of thespallation target Fig.28b Activation of the target coolant per kW of beam at different proton energies per kW of beam at different proton energies
As far as the radiation damage of the target is concerned, the integrated fluxes of high- energy (HE) protons and neutrons have been obtained for different beam energies and reported in Table 7.
Table 7 Integrated flux per kW of beam power for different beam energies
Ta Target HE proton Flux HE neutron Flux sol 10c200 (part/cm2.s) (part/cm2.s) Max Ave Max Ave 140 MeV 1.8x1013 2.7x1012 1.1x1011 2.5x1010 200 MeV 1.3x1013 1.8x1012 1.6x1011 4.3x1010 300 MeV 8.3x1012 1.2x1012 2.1x1011 6.3x1010
The gas production and the displacement rates (dpa/yr) obtained using Eq. (1) together with the particle fluxes listed in Table 7 are reported in Table 8 for the same operating conditions as those listed in section 3.4.4. Table 8 Gas production and the displacement rates per kW of beam
Average Average H He Target 3 HE proton HE neutron Prot. Ener Neut. Ener Production Production (Ta) (dpa/yr) (dpa/yr) (MeV) (MeV) (appm/dpa) (appm/dpa) Max Ave Max Ave 140 MeV 90 51 0.99 54.8 0.6 0.07 0.002 0.001 200 MeV 115 65 2.92 130. 0.5 0.05 0.003 0.001 300 MeV 155 88 6.93 275. 0.4 0.04 0.005 0.002
The particle damage are entirely dominated by the high-energy protons and localized on the inner skin of the spallation target with a maximum situated at the tip of the inner cone. It is reduced with increasing proton energy, but on the other hand, gas production that is responsible for the swelling of the target is drastically enhanced.
3.5.2 Neutron production High-energy neutrons produced by evaporation reactions in the target are more difficult to stop or to attenuate and travel a relatively long distance in the core. Their average energy is about 45 MeV and increases up to 74 MeV, in the case of the most energetic beam. Their spatial distribution is relatively forward peaked and has the tendency to escape through the bottom part of the reactor vessel. In fig.29 and 30 the neutron distribution close to the target and in the core are represented for initial proton energies of 200 and 300 MeV respectively.
Fig.29 High-energy neutron flux distribution (n/cm2/s) per kW of beam at 200 MeV
Fig.30 High-energy neutron flux distribution (n/cm2/s) per kW of beam at 300 MeV The presence of the high-energy neutrons in the core and their anisotropic distribution result in a harder neutron spectrum and higher leakage component, which explains why the neutron multiplication is reduced when the proton beam energy is raised (Table 9). This mitigates the gain in neutron yield and therefore the corresponding reduction of the accelerator intensity. It has to be observed that a good neutron distribution, in this case, will require an upward axial shift of the target of about 20 cm.
Table 9 Variation of the main neutronic parameters with proton beam energy
140 MeV 170 MeV 200 MeV 240 MeV 300 MeV
Neutron Yield 0.80 1.20 1.65 2.34 3.56 (n/p)
Net Neutron 11.7 11.1 11.0 10.4 10.1 Multiplication
k 0.9143 0.9098 0.9093 0.9041 0.9007
Dk/k (pcm) 0.0 -500 -550 -1120 -1500
Scaling Factor 1.0 0.71 0.52 0.38 0.26
These high-energy neutrons penetrate deep inside the reactor core and surrounding biological shield and contribute to the ambient dose inside the reactor building, as shown in Figures 31a and 31b. We estimate the flux of neutrons > 20 MeV reaching the inner most fuel elements to be between 109 and 1010 n/cm2/s per kW of beam and between 107 and 108 in the bottom part of the reactor vessel for incident beams of 200 and 300 MeV respectively.
Fig.31a Ambient dose due to neutrons in Fig.31b Ambient dose due to neutrons in (mSv/h) per mA of beam of 200 MeV protons (mSv/h) per mA of beam of 300 MeV protons 4. Conclusions
The thermo-mechanical issues (progressive plastic deformation and fatigue) becomes less stringent at higher proton energies because the higher neutron yield allows a target power reduction. The neutron multiplication is effective only if the target is axially re-positioned to optimally match with the neutron flux of the TRIGA core. At higher proton energies, the flux levels in the target are reduced together with dpa’s and power densities (easing therefore the cooling capabilities of the target). We note however a considerable increase in the gas release fraction at 300 MeV. The issue of proton stopping becomes more critical and must be faced by a further increase of the flow guide thickness. Since the forced convection of the target has to be kept, to prevent flow disturbances in the TRIGA core, the radial space limitations have to be accounted for. To electronically stop 200 MeV protons, a Ta target thickness of 18 mm. is roughly required; this value is assured by the present target configuration. For higher energy protons it is necessary to develop completely new design based for example on the use of liquid metal. In any case the effects of partial proton stopping by water have to be studied and the acceptability limits, in terms of safety, have to be established.
References
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